A heat exchanger for heat-exchanging two fluids having different temperatures by directly or indirectly contacting the fluids has been widely used in various industrial fields, and especially takes an important role in heating, air conditioning, power generating, exhausted heat recovery and chemical processes.
Especially, a heat exchanger for refrigerating and air conditioning is provided with fins in order to improve heat transfer, as illustrated in FIG. 1. The heat transfer is generated due to low-temperature refrigerants provided in a tube when humid air passes the fins during the heat exchanging operation. When the temperature of the fin surface is lower than a dew point temperature of the humid air, water drops condense on the surface of the heat exchanger, thereby obstructing the air flow, and thus a pressure difference between the heat exchanger's entrance and exit is increased. Therefore, in order to provide an identical flux, blower fan power should be increased, which results in increased power consumption.
In order to solve the problem, a rust resistant process is carried out on the fin of the conventional heat exchanger for providing a corrosion resistant property, a hydrophilicity is provided thereon, and a silicate coating is performed in order to improve a flow of condensed water, which is generally called a pre-coated material (PCM). However, in the PCM manufacturing process, a tetrachloroethane (TCE) for cleansing aluminum and chromium for providing the corrosion-resistance are necessarily used, thereby causing environmental pollution. In addition, the PCM has the excellent hydrophilic property at an initial stage, but with aging gradually loses the hydrophilic property with the lapse of time.
Also, a great deal of chemical goods have been currently employed as a material for wall paper. However, the silicate material for providing the hydrophilic property is volatilized and chemically combined with the wall paper, thereby discoloring the wall paper undesirably.
Efforts have been made to satisfy various demands by forming a functional surface on a material. Among methods known for forming the functional surface are: (1) depositing the functional layer on the surface of the material; and (2) modifying a surface of the material in order to have new physical and chemical properties.
A method for modifying a surface property of a polymer material to hydrophilicity by using an ion beam and a reaction gas has been disclosed by the inventors of the present invention in U.S. Pat. No. 5,783,641. According to this method which is called “Ion Beam Assisted Reaction”, the surface of a polymer material is activated by irradiating energetic argon ions and oxygen ions thereon, and at the same time the surface property of the polymer is modified to hydrophilicity by providing the reactive gas around the polymer and forming hydrophilic functional groups on the surface thereof. In this case, according to “Surface Chemical Reaction between Polycarbonate (PC) and keV Energy Ar+ Ion in Oxygen Environment” (J. Vac. Sci. Tech., 14, 359, 1996) which has been disclosed by the inventors of the present invention, the hydrophilic functional groups, such as C—O, C═O, (C═O)—O, etc., are formed on the surface of the polymer. Many polymers, such as PC, PMMA, PET, PE, PI, and silicone rubber can be modified to have a hydrophilic surface by the ion assisted reaction.
In addition, in accordance with “The Improvement of Mechanical Properties of Aluminum Nitride and Alumina By 1 keV Ar+ Irradiation in Reactive Gas Environment” [“Ion-Solid Interactions For Materials Modification And Processing”, Mat. Soc. Symp. Proc.396, 261 (1996)] which has been disclosed by the inventors of the present invention, the surface modification by the ion beam assisted reaction is a method which can be used not merely for polymer materials, but the surface modification can be also performed on a ceramic material by the ion beam assisted reaction. The characteristics of the ceramic material, such as the mechanical strength thereof can be improved by forming a new functional layer on the surface thereof.
Also, the ion beam assisted reaction can be employed for a metal. When aluminum is processed by the ion beam assisted reaction, the hydrophilicity of the aluminum metal surface is increased. However, the value of the wetting angle with water varied according to the lapse of time on a surface of a process sample which was measured to examine hydrophilicity. That is, the value of the wetting angle increased with the lapse of time, and was restored to its original value after the lapse of a certain amount of time, and thus the effect of the surface modification was only temporary.
When a metal such as aluminum is processed by the ion beam assisted reaction, hydrophilicity is increased because a native oxide layer is removed by etching carried out on the aluminum surface and a functional layer is formed thereon. That is, the effect of improvement in hydrophilicity is reduced with the lapse of time because a native oxide layer is naturally grown on the aluminum surface, and the aluminum surface is restored to its original state because the functional layer which consists of a thin layer (less than several nanometers) has little mechanical resistance against environmental changes (water, temperature, etc.) with the lapse of time.
Accordingly, forming a hydrophilic layer on the surface of the metal by the ion beam assisted reaction which has been utilized for the polymer and ceramic material is ineffective due to the above-described disadvantage.
This disadvantage in modifying the metal material to have hydrophilicity occurs because the hydrophilic layer is not stable. Thus, a hydrophilic layer which is physically and chemically stable should be formed in order to overcome such a disadvantage. A hydrophilic layer which is stable on the metal surface can be formed by depositing a hydrophilic polymer.
In order to deposit a polymer on a material by the conventional deposition technique, at least several process steps are required: (1) synthesizing a monomer; (2) performing a polymerization so as to form a polymer or an intermediate polymer for a next succeeding step; (3) producing a coating solution; (4) cleansing and/or conditioning of a substrate surface by application of primer or coupling agent; (5) coating; (6) drying a coated layer; and (7) curing the coated layer.
The above-described process can be replaced by a one-step plasma polymerization process by introducing a gaseous material to be polymerized into a vacuum chamber under a relatively low vacuum state (10−2–101 Torr), forming a gas plasma by using DC power or RF power, and simultaneously generating a reaction of various ionized gases, radicals and the like which are formed inside the plasma under the applied energy. To form a polymer and depositing same on a substrate, the polymer formed according to the plasma polymerization has strong adhesion to the substrate and high chemical resistance.
For example, the plasma polymerization may be performed on the metal surface according to the technique disclosed in U.S. Pat. No. 4,980,196. A low-temperature plasma process is employed so as to prevent corrosion of a steel, the process including the steps of: (1) pretreating the steel substrate by a reactive or inert gas plasma; (2) using DC power from 100–2000 volts, preferably 300–1200 volts for the plasma deposition; (3) making the steel substrate the cathode; (4) having anode(s) equipped with magnetic enhancement (i.e. magnetron); and (5) using organosilane vapors (with or without non-polymerizable gas) as the plasma gas to be deposited. That is, in accordance with U.S. Pat. No. 4,980,196, the cathode is used as the substrate, and a magnetron is installed on the anode. The plasma is formed on the steel substrate by using the organosilane vapors and DC power. The plasma polymerization is then carried out. In addition, the above-described patent further discloses performing a primer coating after the plasma polymerization.
However, a magnetron must be installed at the anode side to perform the above-described process, and thus the device is more complicated. There is another disadvantage to the process in that the degree of hydrophilicity or hydrophobicity cannot be controlled.